I'm reporting my experience as well hoping it'll help with the issue.
I use Band in a Box on two macs: a MBP15" 2018 with 16GB RAM with a 4GB Radeon Vega 20 and an iMac Pro with a 10-core Xeon, 64GB RAM, and a 16GB Radeon Vega 64. Both are running the latest version of Big Sur at the time of writing: 11.1.
I have upgraded BiaB to V421 on both computers. The program runs on the laptop, but crashes on the iMac. Disabling the plugins by putting the safemode.txt file in the preference directory makes the program run on the iMac. In both systems I see the purple streaks, regardless of whether or not the interface is compact or full toolbar. The purple streaks do not affect the rest of the functionality.
The only difference between the two is that on the iMac Pro I have installed the Krisp microphone enhancer, whereas it is not installed on the laptop.
I use BiaB just for personal enjoyment, as a complement to my arranger keyboard, so the fact that it does not work perfectly at this time is not causing any immediate or critical issue. I'd have liked to use BiaB a bit more during the holidays, but that's not a serious issue.
The combination of changes in the OS that create unwanted side effects, the possible technical debt accumulated on development, and the raising priority for the 2021 Mac release creates a difficult situation for everyone, which is unfortunate.
I hope that by contributing with our individual cases, we can provide an ampler database that helps resolve the issue for both the 2020 version and the 2021 version of BiaB for Mac. I also hope that the team can have the time to consider reducing the technical debt in the UI, even though I understand that using cross-platform user interface libraries creates inevitable restrictions. In my dreams, I'd imagine a version of BiaB that fully adopts SwiftUI.
In conclusion, my thanks to the development team for an excellent program and to the music team for the great artists and ideas embedded in BiaB. I hope this helps, and I look forward to the next versions; I will continue to support you as much as I can.
All the best,